It is often desirable to use personal listening devices when listening to music and other audio material, or when participating in a telephone call, in order to not disturb others that are nearby. When a compact profile is desired, users often elect to use in-ear earphones or headphones, sometimes referred to as earbuds. To provide a form of passive barrier against ambient noise, earphones are often designed to form some level of acoustic seal with the ear of the wearer. In the case of earbuds, silicone or foam tips of different sizes can be used to improve the fit within the ear and also improve passive noise isolation.
With certain types of earphones, such as loose fitting earbuds, there is significant acoustic leakage between the atmosphere or ambient environment and the user's ear canal, past the external surfaces of the earphone housing and into the ear. This acoustic leakage could be due to the loose fitting nature of the earbud housing, which promotes comfort for the user. However, the additional acoustic leakage does not allow for enough passive attenuation of the ambient noise at the user's eardrum. The resulting poor passive acoustic attenuation can lead to lower quality user experience of the desired user audio content, either due to low signal-to-noise ratio or speech intelligibility especially in environments with high ambient or background noise levels. In such a case, an ANC mechanism may be effective to reduce the background noise and thereby improve the user's experience.
ANC is a technique that aims to “cancel” unwanted noise, by introducing an additional, electronically controlled sound field referred to as anti-noise. The anti-noise is electronically designed so as to have the proper pressure amplitude and phase that destructively interferes with the unwanted noise or disturbance. An error sensor (typically an acoustic error microphone) is provided in the earphone housing to detect the so-called residual or error noise. The output of the error microphone is used by a control system to adjust how the anti-noise is produced, so as to reduce the ambient noise that is being heard by the wearer of the earphone.
The amplitude and phase characteristics of the anti-noise needed for achieving effective noise control are a result of processing the noise, as captured by one or more sensors, through a control filter. An ANC system in general can be implemented in a feedback or a feed forward topology, or a hybrid topology. Generally, the control filter processes ambient noise content that has been measured or is contained in the output of a sensing microphone (for example, the error microphone and in some cases also a reference microphone). The control filter does so based on an assumption that a certain electroacoustic response exists between the earphone speaker driver and the error microphone, when the earphone has been placed in or against the ear. This electroacoustic response is often referred to as the plant S, or the secondary acoustic path transfer function S(z), where this reference is in view of a primary acoustic path P(z) that is taken by the disturbance in arriving at the error microphone. In a feedback type of ANC system, a signal representing the residual error (reflecting the disturbance) as picked up by the error microphone is fed to the control filter, which in turn produces the anti-noise. The control filter is intended to create an anti-noise that destructively interferes with the disturbance that has arrived at the error microphone through the primary acoustic path. In a feed forward system, the input signal to the control filter is derived from the output of a reference microphone which is located so as to pickup the disturbance before the disturbance has passed through the primary acoustic path. In a hybrid approach, elements of the feed forward and feedback topologies are combined to produce an anti-noise based on both an output of the reference microphone and an output of the error microphone.